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Labels on a cereal box, a can of soup, a package of prepared dinner all contain the percentage of nutrient requirements provided by the products. This information implies that the nutrient does something important, that it provides an essential function. What are some of these functions?

The need for the major nutrients--carbohydrates, fats and proteins--is the easiest to understand. Our bodies use carbohydrates--mainly sugars and starches--primarily to supply energy, and also to build supporting structures of cells and tissues. We also use fats for energy. Plus, they are essential structural components of cell membranes and precursors of steroid hormones and other signaling molecules. Proteins provide the amino acids for making structural proteins, as well as enzymes and peptide hormones. Even proteins provide energy as they are broken down, although this is not a preferred source.

Our requirements for vitamins and minerals are more subtle but every bit as essential as those of the major nutrients. Vitamins usually act as coenzymes, helping enzymes do their job of building molecules and generating energy from carbohydrate, fat and protein fuels. For instance, the B vitamins thiamine, niacin and riboflavin are involved in energy use. Vitamin B12 and folic acid are important in gene replication and hence in growth and development.

Other vitamins have more specific functions: Vitamins C and E prevent oxidative damage; vitamin A contributes to the light sensing pigment in the eye; vitamin D controls absorption of calcium through the intestines and its insertion into bone; and vitamin K regulates formation of clotting factors.

Essential minerals also have diverse functions. In the body, minerals generally exist as electrically charged particles called ions. These ions may help large molecules adhere to one another or form shapes appropriate to their function. For example, calcium is necessary to the structure of bone; zinc is a component of receptors in the cell membrane that respond to estrogen and other hormones; and iron enables hemoglobin to properly bind and transport oxygen in the blood.

Because of their electrical nature, mineral ions--primarily sodium, potassium and calcium--maintain charges across membranes and carry electrical current in excitable tissues such as nerves and muscles. Without these minerals, our brain, nerves, muscles and heart simply would not work.

Minerals may also be necessary for enzymes to function, either as integral parts of enzymes or as enzyme activators. For instance, copper is an essential part of an enzyme that makes collagen--the main component of tendons--as well as an enzyme that makes a form of the hormone adrenaline. Zinc is part of an enzyme that produces a hormone that regulates blood pressure. Certain antioxidant enzymes require copper, selenium, iron and manganese to function.

Finally, minerals may serve as chemical signals. Generally they do so by some form of enzyme activation. For example, calcium activates enzymes in heart and muscle to initiate contraction, in blood vessels to cause dilation, and in secretory organs to cause secretion. Minerals are also part of molecules needed to trigger changes in gene expression. They do this by binding to proteins that interact with genes to start producing structural proteins, hormones and enzymes.

Thus, the information on the food label is aimed at helping us to make sure that we consume the proper amounts of nutrients to support these and many other important functions.